Why Millionaires Prefer Dogs Over Cats

Dogs and millionaires have a lot in common. They are relentless opportunists (especially when it comes to rewards). They come in all shapes, sizes and colors. They defend their turf. And in general, they don't like cats.

Perhaps that explains a new survey that shows that millionaires are far more dog-friendly than the rest of Americans.

According to a study by Spectrem Group, 58 percent of millionaire pet owners have a dog. Only 37 percent own a cat. Only 3 percent keep fish, 2 percent birds and 2 percent have a horse.

Those stats are far more canine-leaning than the rest of the country. According to the Humane Society, 39 percent of U.S. households own a dog, compared to 33 percent of households that own a cat. (Read more: The Top Three Investments Among the Rich)

Why have millionaires gone to the dogs?

Jennifer Cona, a trust and estates attorney and partner with Genser Subow Genser & Cona in New York, does a lot of work on pet trusts. She said that of all the pet trusts she's worked on, 90 percent are for dogs and only 10 percent are for cats. (She's written only one parakeet trust).

She said dogs provide one thing especially important to the wealthy: "unconditional love."

"You don't get that from a cat," she said. "Dogs are like children for some families, except they don't mess up in college or run off with money. Sometimes it's easy to see why dogs are the favorite child."

Plus, she says, millionaires know that dogs don't love them for their money.

Millionaires show their love for their dogs in part by their spending. One quarter of millionaire pet owners spend more than $1,000 a year on their pets every year, the Spectrem study said, while more than half spend more than $500 a year. (Read more: When $20 Million Is Too Much to Leave the Kids)

Many would say those numbers are understated, given all the diamond-dog collars, chateaubriand dog foods and booming dog spas in evidence these days. Not to mention the medical bills.

The survey showed that 34 percent of pet owners spend money on grooming, while only six percent spend on "sweaters, accessories, outfits and costumes."

More than half of millionaire pet owners spend money on teeth cleaning for their pets. More than 16 percent, meanwhile, said they would spend money on reconstructive knee surgery, hip surgery and "anti-anxiety, anti-depression" medication for their pets.